(1) Field of the invention
The present invention relates to system and method for controlling an induction motor which is, for example, used for an electric motor-driven vehicle and which achieves a high efficiency drive of the induction motor.
(2) Description of the background art
A Japanese Patent Application First (unexamined) Publication No. Showa 61-121783 published on Jun. 6, 1986 exemplifies a system for controlling an (AC, squirrel-cage) induction motor applied to an electric (motor-driven) vehicle in which a power regenerated by driving the induction motor from a load during a braking of the electric vehicle is returned to its power supply of the motor (regeneration).
The control system disclosed in the above-identified Japanese Patent Application First Publication includes means for generating an instruction signal of a slip frequency and means for calculating a current instruction value to drive the induction motor on the basis of the instruction signal of the slip frequency. In the control system described above, different target values of the instruction signal of the slip frequency are generated during a power drive state in which the load is driven from the induction motor and during a regenerative state in which the induction motor, in turn, is driven from the load so that a transition from the power drive state to the regenerative state can slowly be carried out. Consequently, the system can prevent an abrupt torque variation from being generated during the transition.
For example, suppose that a rotor magnetic flux of the motor is denoted by .omega..sub.r, a torque current instruction value is denoted by i.sub.T, an output torque of the motor is denoted by T.sub.e, the slip frequency is denoted by .omega..sub.se, the power supply frequency is denoted by .omega., a motor revolution number per time (electric angle) is denoted by .omega..sub.re, and constant numbers are denoted by k.sub.1 and k.sub.2.
Then, the following equations are established: EQU .omega..sub.se =.omega.-.omega..sub.re =k.sub.1 .multidot.i.sub.T /.phi..sub.r EQU T.sub.e =k.sub.2 .multidot..phi..sub.r .multidot.i.sub.T.
As appreciated from the above equations, a desired regenerative torque is generated by varying the torque current instruction value i.sub.T with the magnetic flux .phi. as constant in a case when the instruction value of the slip frequency .omega..sub.se is given in the above equations. In this way, the torque control of the disclosed system is carried out by specifying the slip frequency .omega..sub.se.
In this way, the induction motor is driven according to the torque current instruction value determined corresponding to the slip frequency instruction signal set irrespective of an energy loss (efficiency).
On the other hand, another Japanese Patent Application First Publication No. Heisei 2-23085 published on Jan. 25, 1990 (hereinafter referred to as reference A) exemplifies a control method for the induction motor.
In the reference A, an iron loss resistance R.sub.M is newly added to a d-axis component of an induction motor model expressed in a d-q coordinate system so as to describe a loss in the induction motor (iron loss, copper loss). Then, the induction motor is driven so as to minimize these losses in a steady state.
Specifically, a generally known vector control is used to control the torque of the induction motor according to the slip frequency .omega..sub.se derived from a condition of minimization of the steady-state loss. Since the iron loss resistance R.sub.M is varied according to a motor revolution speed, the slip frequency .omega..sub.se-opt which minimizes the loss can be derived by reading a table map of the iron loss resistance R.sub.M from the motor revolution speed and by solving an equation describing the condition of minimization of such a loss as described above.
However, in the induction motor control method disclosed in the reference A, since the condition of minimization of loss is derived from a steady-state characteristic of the motor and no consideration of a transient characteristic of the motor is given thereto, the loss of the motor torque during a time of transient response is not minimized.